<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{width: 200px;
            height: 200px;
            background-color: green;
            background-color: rgba(0, 128, 0, .3);
            /* opacity: 0.3; */

}
    </style>
    
</head>
<body>
    <div>
        123
    </div>
    
</body>
</html>